1. Technical Field
The present invention relates to an electro-acoustic transducer and an electronic device, and more particularly, to an electro-acoustic transducer used in a home audio, and an electronic device, for example, an audiovisual device such as an audio set, a personal computer, a television, and the like, which includes the electro-acoustic transducer.
2. Background Art
Recently, media such as DVD, DVD-AUDIO, and the like have been popularized, and there is a desire for an electro-acoustic transducer having a high reproduction band in order to reproduce extremely high frequencies included in their contents. To achieve the reproduction of the extremely high frequencies, there have been proposed electro-acoustic transducers as shown in FIGS. 24 and 25 (e.g. refer to Patent Documents 1 and 2). FIG. 24 is a cross-sectional view showing a configuration of a conventional electro-acoustic transducer 91. FIG. 25 is a cross-sectional view showing a configuration of a conventional electro-acoustic transducer 92.
As shown in FIG. 24, the electro-acoustic transducer 91 includes a yoke 911, a magnet 912, a diaphragm 913, and drive coils 914a and 914b. The yoke 911 is a member having a recessed shape, and formed of magnetic material such as iron, or the like. Side portions of the yoke 911 extend upward so as to be perpendicular to a bottom thereof. The magnet 912 is a neodymium magnet which is polarized in an up-down direction. The magnet 912 is a columnar body. The magnet 912 is fixed to an inner bottom surface of the yoke 911. Between side surfaces of the magnet 912 and inner side surfaces of the yoke 911 are respectively formed magnetic gaps G1 and G2 which have the same width. An upper surface of the magnet 912 is flush with upper surfaces of the side portions of the yoke 911. The diaphragm 913 is fixed to the upper surface of the magnet 912 and the upper surfaces of the sides of the yoke 911. The drive coil 914a is fixed to an upper surface of the diaphragm 913 so as to be located in or adjacent to the magnetic gap G1. The drive coil 914b is fixed to the upper surface of the diaphragm 913 so as to be located in or adjacent to the magnetic gap G2.
A magnetic pole at the upper surface of the magnet 912 is assumed to be a north pole. At this time, a magnetic flux emitted from a central portion of the upper surface of the magnet 912 is emitted vertically and upwardly from the upper surface of the magnet 912, and extends vertically and downwardly through the drive coils 914a and 914b. On the other hand, a magnetic flux emitted from an outer peripheral portion of the upper surface of the magnet 912 spreads radially from the upper surface of the magnet 912, and extends obliquely and downwardly through the drive coils 914a and 914b. When a current flow through the drive coils 914a and 914b in such a magnetic field, driving forces in the up-down direction are generated in the drive coils 914a and 914b, respectively. The driving forces vibrate the diaphragm 913 in the up-down direction.
As shown in FIG. 25, the electro-acoustic transducer 92 includes a lower casing 921, an upper casing 922, a first magnet 923, a second magnet 924, a diaphragm 925, and a drive coil 926. The lower casing 921 and the upper casing 922 are box-shaped members, and formed of non-magnetic material. The lower casing 921 and the upper casing 922 are combined to form a casing. The first and second magnets 923 and 924 are cylindrical bodies. The first magnet 923 has the same outer diameter as that of the second magnet 924. The first magnet 923 is fixed to an inner upper surface of the upper casing 922. The upper casing 922 is formed with openings 922h at a part of a bottom thereof, to which the first magnet 923 is not fixed. The second magnet 924 is fixed to an inner bottom surface of the lower casing 921. The first magnet 923 has a central axis which coincides with that of the second magnet 924. The first magnet 923 is polarized in an up-down direction. The second magnet 924 is polarized in the up-down direction but in a direction opposite to the polarization direction of the first magnet 923. The diaphragm 925 is fixed at an outer peripheral portion thereof to the lower casing 921 and the upper casing 922 so that the outer peripheral portion thereof is interposed between the lower casing 921 and the upper casing 922. The drive coil 926 is fixed to an upper surface of the diaphragm 925 so as to include a line connecting an outer periphery of the first magnet 923 to an outer periphery of the second magnet 924.
When a magnetic pole at a lower surface of the first magnet 923 is assumed to be a north pole, a magnetic pole at an upper surface of the second magnet 924 is a north pole. Thus, a magnetic flux emitted vertically and downwardly from the lower surface of the first magnet 923 bends substantially at a right angle to become a horizontal magnetic flux. Similarly, a magnetic flux emitted vertically and upwardly from the upper surface of the second magnet 924 bends substantially at a right angle to become a horizontal magnetic flux. When a current flows through the drive coil 926 in such a static magnetic field, a driving force in the up-down direction is generated in the drive coil 926. The driving force vibrates the diaphragm 925 in the up-down direction to emit sound from the diaphragm 925. The sound emitted from the diaphragm 925 is released through the openings 922h to the outside.
[Patent Document] Japanese Laid-Open Patent Publication No. 2001-211497
[Patent Document] Japanese Laid-Open Patent Publication No. 2004-32659
In the conventional electro-acoustic transducer 91 shown in FIG. 24, however, the magnetic flux parallel to the vibration direction is more dominant than the magnetic flux perpendicular to the vibration direction. The driving forces generated in the drive coils 914a and 914b are proportional to a magnetic flux in a direction perpendicular to the direction of the current flowing through the drive coils 914a and 914b and the vibration direction of the diaphragm. In other words, the driving forces are proportional to a magnetic flux in a direction perpendicular to the vibration direction. Thus, in the conventional electro-acoustic transducer 91 shown in FIG. 24, since the magnetic flux parallel to the vibration direction is more dominant, sufficient driving forces cannot be obtained. As a result, there is a problem that a sound pressure level of reproduced sound is lowered.
Further, the conventional electro-acoustic transducer 91 shown in FIG. 24 includes only the one magnet 912. Here, there are considered a case where the diaphragm 913 is vibrated upwardly (in a direction to separate from the magnet 912) from an initial state where a current does not flow though the drive coils 914a and 914b, and a case where the diaphragm 913 is vibrated downwardly (in a direction to approach the magnet 912) from the initial state. A magnetic flux emitted from a magnet decreases in proportion to a distance from the magnet. Thus, the magnetic fluxes extending through the drive coils 914a and 914b are different in magnitude from each other in each of the cases. In other words, driving forces generated in the drive coils 914a and 914b are different from each other depending on the vibration direction. As a result, in the conventional electro-acoustic transducer 91 shown in FIG. 24, there is a problem that asymmetric nature of the magnetic fluxes causes distortion of the driving forces thereby deteriorating a quality of the reproduced sound.
In addition, the conventional electro-acoustic transducer 92 as shown in FIG. 25 includes, in addition to the second magnet 924, the first magnet 923 for increasing the magnetic flux in the direction perpendicular to the vibration direction to obtain a sufficient driving force. However, the first magnet 923 is located on a sound emission surface side with respect to the diaphragm 925. Thus, the first magnet 923 becomes an acoustic load with respect to the sound emitted from the diaphragm 925. The first magnet 923 has the same outer diameter as that of the second magnet 924. Thus, in the conventional electro-acoustic transducer 92 shown in FIG. 25, there is a problem that an effect of the acoustic load by the first magnet 923 is significant, thereby deteriorating a quality of reproduced sound. Particularly in an extremely high frequency band equal to or higher than 20 kHz, the quality of the reproduced sound is significantly deteriorated by the acoustic load.
Therefore, an object of the present invention is to provide an electro-acoustic transducer and an electronic device which are capable of reproducing high-quality sound while increasing a driving force generated in a drive coil and preventing deterioration of a sound quality due to distortion of the driving force.